test: dm: add MDIO test

A very simple test for DM_MDIO, mimicks a register write/read through the
sandbox bus to a dummy PHY.

Signed-off-by: Alex Marginean <alexm.osslist@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
diff --git a/arch/Kconfig b/arch/Kconfig
index 355d214..a946af8 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -123,6 +123,8 @@
 	imply DM_SOUND
 	imply PCI_SANDBOX_EP
 	imply PCH
+	imply PHYLIB
+	imply DM_MDIO
 
 config SH
 	bool "SuperH architecture"